1. The Field of the Invention
The present invention relates to an apparatus for detecting a concentration of a gas
2. Related Art
A gas concentration detecting apparatus has many types depending on applications. One type, as disclosed by Japanese Patent Laid-open (KOKAI) publication No. 2002-20285, is an apparatus that employs a limiting current type of gas concentration sensor in order to, for example, detect NOx (nitrogen oxide) included in an exhaust gas discharged from an automobile engine. In this apparatus, the gas concentration sensor is structured, as an example, to have three cells consisting of a pump cell, a sensor cell, and a monitor cell. The pump cell is responsible for discharging and drawing oxygen in an exhaust gas taken into a chamber, during which time the pump cell detects an oxygen concentration in the exhaust gas. The sensor cell detects a NOx concentration (i.e., a concentration of a particular gas component) from the gas that has passed the pump cell, and the monitor cell detects a concentration of oxygen that is residual in the chamber after the gas has passed the pump cell.
In detail, the detection of the NOx concentration can be performed such that a censor cell current is measured in response to application of voltage to the sensor cell, and a signal indicative of the measured current is fed to a microcomputer via an A/D converter. The microcomputer converts the digitized current signal to a corresponding concentration, resulting in acquisition of the value of the NOx concentration.
For using the gas concentration sensor as an exhaust gas sensor, a signal indicating the value of the NOx concentration is, for example, outputted to an engine ECU controlling the engine. The engine ECU uses the signal to control the engine for the purpose of absorbed NOx is deoxidized and discharged with the use of a NOx occlusion-deoxidization type catalyst and uses the signal as information indicative of failure diagnosis of sensors or others under the regulations such as OBD. In such a case where the regulations such as OBD is effective, it is necessary to detect a NOx concentration in a concentration detecting range whose range is wider than that used for the normal control. Thus, the detection range for the NOx concentration and others are previously determined in accordance with the regulations.
However, as described above, in cases where the NOx concentration should be given to plural applications and the regulatory compliance forces the concentration detecting range to be extended, resolution for the NOx concentration becomes rough, because a microcomputer or other processing apparatuses are usually limited in a processing facility. When such applications are directed to the regulations such as OBD, any problem will not arise even if the resolution for the NOx concentration is relatively rough. In contrast, for applications such as engine control, such a limited processing capability may cause a problem that a desired concentration detecting accuracy cannot be met. Recently, there is a tendency that the regulations for emission of the exhaust gas have been strengthened increasingly. It is therefore desired to overcome these situations.